X-rays are produced in an X-ray tube as a result of high speed electrons striking a target material. The electrons strike and penetrate the surface layers of the target material and through interaction or collision with the atoms of the target, the energy of the electron is imparted to the electrons in the target.
If, in striking the target, the energy of the electron is dissipated through a series of collisions with the outer electrons of the target atoms, then the energy is released either in the form of heat or as visible light. An electron may, after a series of collisions, also emerge from the target as a back-scattered electron. These collisions result in most of the energy losses contributing to target heating and hence reduced X-ray tube life.
The electron may also have radiative collisions, giving up part or sometimes all of its energy to photons. The photons produced as a result of these collisions have an energy less than or equal to the energy given up by the electron.
If the energy of the electron is sufficient to collide with and eject an electron from the inner K-shell of the target atom, then the excited target atom, when the electrons in the outer shells drop into the vacant inner shell, will return to its ground state and a photon will be emitted. The energies of these transitions are dependent upon the atoms comprising the target material and hence the energies of the photons emitted are characteristic of the target atom. This radiation is known in the art as the characteristic X-ray radiation and is produced by the X-ray tube only when the energy of the electron striking the target is above the level required to dislodge the K-electron of the target atom.
The energy of the photon comprising the X-ray is directly related to the energy given up by the electron in the collision with the target molecules. As it is well known that the relationship between the wavelength (.lambda.) of a photon and its energy is expressed by the Duane-Hunt equation: ##EQU1## this process results in X-rays of various wavelengths which constitute what is known in the art as the continuous X-ray spectrum.
The ability of the X-rays to penetrate an examination object depends on the wavelength or energy of the X-ray photons as well as the composition of the examination object - i,e. its chemical elements, thickness and density. With respect to the wavelength or energy of the X-rays, generally the penetration ability is inversely proportional to wavelength or directly proportional to energy. Thus, short wavelength (high energy) X-rays have a greater penetrating ability than long wavelength (low energy) X-rays. With respect to the chemical elements making up the examination object, generally, the higher the atomic number of the element, the less the penetration of the X-ray beam. However, at wavelengths or energy levels near the absorption edges of the elements, these generalizations do not hold true as there are discontinuities in the degree of absorption of the X-ray beam at these points. With respect to the thickness and density of an examination object, generally, the thicker and denser the object the greater its ability to absorb X-rays and thus fewer X-rays pass through the object. It is the combination of these factors as they relate to different compositions of material which allows for the differential diagnosis of radiography. Thus, the selection of the operating parameters of the X-ray apparatus during medical diagnosis depends upon the examination object, its chemical composition, thickness and density. For more descriptions of the above, reference can be made to textbooks of medical physics or radiology.
As low energy X-rays do not normally contribute to the resolution of the method but are merely absorbed and scattered by the examination object, it is highly desirable to remove such X-rays from the X-ray beam prior to the beam contacting the examination object. These low energy X-rays are usually removed from the X-ray beam through the use of attenuators or filters.
Similar to the effects on examination objects, the attenuating ability of a filter is dependent upon the chemical composition, density and thickness of the material making up the filter. These relationships are represented by the following equation: EQU I=I.sub.o e.sup.-.mu.ox
where I is the intensity of the radiation transmitted, I.sub.o is the intensity of the incident radiation, e is the base of natural logarithm, .mu. is the mass attenuation coefficient for the chemical element comprising the filter material, .rho. is the density of the filter material, and x is the thickness of the filter material.
Of the above factors, all except the attenuation co-efficient .mu. are independent of the frequency or energy of the incident radiation. The attenuation co-efficient varies with the energy of the incident radiation and is related to the atomic number of the chemical element of the filter material. These co-efficients have been experimentally determined and can be found in published tables, such as, for example, in UCRL 50174 by W.H. McMaster et al available from the National Technical Information Services, Springfield, Va., 22151.
For many years the most common means of filtration of X-rays used in medical and dental diagnosis has been through the use of aluminum filters. As an example, U.S. Pat. No. 2,225,940 discloses a wedge which is brought into the path of the X-ray beam. Additionally, U.S. Pat. No. 3,976,889 discloses the use of variable thicknesses of aluminum filters in dental x-rays to vary exposure levels. Almost all commercial x-ray units have some inherent filtration equivalent to about 1.0 to 1.5 mm of aluminum and those designed for medical and/or dental applications, utilize additional aluminum filtration.
The use of filters other than aluminum to filter low energy X-rays from an X-ray beam was the subject of U.S. Pat. No. 4,499,591, wherein a 127 micron thick yttrium filter was employed to filter the X-ray beam such that energies below 20 keV were eliminated from the beam. Also Heinrick and Schuster, "Reduction of Patient Dose by Filtration in Pediatric Fluoroscopy and Fluorography" Ann. Radiol. (1976) Vol. 19. pages 57-66, utilized a molybdenum filter of 100 microns to remove radiation below 20 keV from the X-ray beam.
Koedooder and Venema; Phys. Med. Biol. (1986) Vol , pages 585-600 describe a computer program which was developed to calculate possible filter materials for use with a range of kVP values and different image receptors. In their results they found that dose reductions of up to 40% were achievable, however, in most cases the loading of the X-ray tube was doubled resulting in reduced life of the X-ray tube.
In X-ray crystallography and diffraction studies, it is useful to have relatively homogeneous, monochromatic X-ray beams. Filter materials have been used for producing these relatively homogeneous X-ray beams by limiting the range of wavelengths of the X-ray beam. Thus, in U.S. Pat. No. 1,624,443, the use of a filter with a slightly lower atomic weight than the X-ray tube target has been found to produce an X-ray beam of suitable relative homogeneity for use in X-ray crystallography. This patent discloses, in a preferred embodiment, the use of a zirconium filter with a molybdenum target. The use of filters of the same material as the target has also been shown to result in an X-ray beam of relative homogeneity. U.S. Pat. No. 3,515,874 discloses the use of molybdenum for both a target and filter, particularly for mammography where it has been found that the energy level of the K.alpha. line emitted from a molybdenum target is ideal for resolution of tumors in mammography applications.
As seen from the above, it is appreciated that there is a risk involved when dealing with diagnostic X-rays due to the harmful effects of unnecessary radiation dosages. Therefore, there is a need for an efficient X-ray filter to reduce such dosages and which is compatible with existing X-ray equipment.